This invention relates to an abrasion-resistant tire tread rubber composition comprising halogenated isobutylene rubber.
Butyl rubbers are copolymers of isobutylene with a conjugated diene, usually isoprene, which provides double bonds allowing the rubber to be vulcanized with sulphur and other vulcanizing agents. On adding a halogen such as chlorine or bromine to butyl rubber in an inert solvent like hexane, a rapid electrophilic substitution reaction occurs, whereby one halogen atom is added per isoprene unit, mainly in the allylic position. The addition of a second halogen atom proceeds at a much slower rate, and the addition of halogen atoms to the double bonds occurs hardly at all. Thus, only a relatively small number of halogen atoms are built into the polymer chain, and a typical brominated butyl rubber comprises 1 to 3.5% bromine.
Though the properties of butyl rubber and halogenated butyl rubber are similar, the halogen atom and the allylic halogen structure in halogenated butyl rubber results in an enhanced cure reactivity. Vulcanization rates are faster with halogenated butyl rubber and require lower amounts of curatives, and, for the same cure system, higher states of cure are possible. Bromobutyl rubber compounds also give better adhesion to other rubbers. Because of these properties, bromobutyl rubber vulcanizates have lower gas permeability, better weather and ozone resistance, higher hysteresis, and better chemical and heat resistance than their butyl rubber counterparts. Bromobutyl rubbers have been employed in innerliners of tubeless tires, innertubes for demanding applications, tire components, linings, belts, hoses and the like. (For a brief summary of production, properties and uses of bromobutyl and chlorobutyl, see Hofmann, Rubber Technology Handbook, Oxford University Press, 1989, pages 92 to 93, and Heinisch, K. F., Dictionary of Rubber, J. Wiley & Sons, 1974, pages 64 to 65.)
Though halogenated butyl rubbers also exhibit high damping characteristics, the rubbers have not been employed in pneumatic tire treads for traction enhancement because of poor tear and abrasion resistance. Desirable tire tread compositions are engineered to produce traction, speed, and stability, simultaneously providing frictional contact for the transmission of driving, braking and cornering forces as well as wear resistance. Typical tread rubber compositions employ elastomers having a relatively high tensile strength and high abrasion resistance.
For example, Studebaker and Beatty described three tread compositions: two had styrene/butadiene elastomers and one had natural rubber (Eirich, F. R., Science and Technology of Rubber, Academic Press, 1978, pages 374 to 375; compounds 188, 208 and 211 are treads). The example tire tread rubber of the Bhakuni, et al. review (Encyclopedia of Polymer Science and Engineering, 2nd ed., volume 16, John Wiley, 1989, page 861) contained styrene/butadiene and butadiene rubber. Takino, et al., disclosed a tire tread rubber composition comprising isoprene and styrene/butadiene in a ratio of 5/95 to 60/40 and 50 to 200 parts by weight carbon black, with the isoprene component containing more than 50% 3,4-vinyl and 1,2-vinyl bonds and the styrene/butadiene rubber having a glass transition temperature lower by 10.degree. C. or more than the isoprene rubber (U.S. Pat. No. 4,824,900).